The study is an NSABP sponsored multi-center randomized trial to determine if tamoxifen in a daily dose of 20 mg reduces the risk of developing breast cancer compared to placebo. Women eligible for the study include women over the age of 60 or women 35 to 59 who have an estimated risk of developing breast cancer similar to that of a 60 year old woman. Participants take study pills for 5 years. We stopped enrolling women to the study in 1993; NSABP closed the study to enrollment in 1997, and unblinded the study in April of 1998.